1. Field
Embodiments of the present invention generally relate to methods and systems for simultaneous communication with multiple wireless communication devices.
2. Description of the Related Art
Wireless local area networks (WLAN)s provide means of transferring data between access points (AP)s and client devices using wireless signaling as defined in the IEEE 802.11 standards. The initial WLAN standards (802.11 a/b/g) were built upon the assumption that only one AP or client device could transmit on a specific channel at a time and the transmitting device would transmit only one set of information at once. Later, the 802.11n standard introduced methods for transmitting multiple information sets simultaneously using a technique known as multiple input multiple output (MIMO), but still with the assumption that only one device could transmit at a time and the information sets would be destined to only one client device or a group of client devices all receiving the same information. Later, the 802.11ac standard introduced methods of permitting an AP to transmit multiple sets of information simultaneously with each information set destined for different client devices using a technique referred to as downlink (DL) multi-user MIMO (MU-MIMO), but still with the limitation that only one device could transmit at once. The advancements in 802.11 technologies have led to peak over the air data rates of up to 6.9333 gigabits per second (Gbps) while utilizing 160 MHz of bandwidth in the 802.11ac standard.
For all these advances in 802.11 technologies, however, WLAN APs are frequently limited in practice to providing under 30 megabits per second (Mbps) of total throughput in many real world scenarios with a high density of client devices. There are many reasons for this practical limitation in throughput in high density scenarios. Firstly, 802.11 uses a distributed scheduling mechanism in which the AP and client devices individually attempt to determine appropriate times to access the wireless medium. If multiple AP or client devices transmit simultaneously, the information is generally lost because the signals interfere. WLAN devices compensate for this by accessing the medium less frequently in an attempt to reduce the probability of additional data loss. This results in an inefficient and frequently unreliable method of sharing the medium in high density environments. Secondly, the MU-MIMO techniques available for WLAN devices are limited in usefulness for solving the primary problems in high density environments. 802.11 supports no uplink (UL) MU-MIMO mechanism which would allow the AP to receive from multiple client devices simultaneously. Although UL MU-MIMO is available in other technologies such as LTE (3GPP TS 36), a similar implementation would require fundamental changes to the 802.11 standard and devices. DL MU-MIMO which is supported by 802.11ac requires frequent updates on the channel between the AP and client devices in order to create a DL MU-MIMO transmission. The gathering of channel information to support DL MU-MIMO happens frequently due to the rapidly changing channel. Without any UL MU-MIMO technique to allow the AP to quickly gather this information, the AP must gather this information from each device individually resulting in decreasing efficiency as more devices are added to the DL MU-MIMO transmission and resulting in a practical limit to the effectiveness of DL MU-MIMO. For these reasons and more, the throughput and reliability of WLAN is generally problematic in high density scenarios. In summary, most of the problems and limitations of WLAN in environments with a high density of client devices stems from the inability to receive multiple WLAN signals simultaneously.
The prevalent implementations of 802.11 utilize orthogonal frequency division multiplexing (OFDM) in which many symbols are transmitted simultaneously with each simultaneously transmitted symbol utilizing a different frequency. The individual frequencies used to transmit the different symbols are referred to as subcarriers. OFDM generally relies on the average signal to interference plus noise ratio (SINR) at the individual subcarriers to be sufficient to permit proper channel estimation and subsequent demodulation. In the case where multiple 802.11 transmissions occur simultaneously and with no prior knowledge of the channel to resolve the transmissions, information from multiple sources contributes to the subcarrier. Given that 802.11 has no inherent mechanism for resolving the information on a subcarrier when the information arises from multiple sources, the information from all sources is generally lost.
One method for overcoming this limitation is for a receiving device with multiple antennas to know what devices will be transmitting, know the change the channel will induce on each subcarrier for the various devices at each of the antennas, and determine appropriate subcarrier weighting vectors to apply to the received signals to isolate each of the various signals. Such a mechanism is available within LTE (3GPP TS 36) in which the base station device can explicitly control when devices are permitted to transmit and has mechanisms for getting channel estimates from the client devices of interest. LTE further enforces tight frequency alignment for devices, tight timing control over devices, and has signal properties permitting the base station to readily gather channel estimates from multiple client devices simultaneously. 802.11, conversely, has a shared scheduling mechanism in which devices seemingly transmit at random. Furthermore, 802.11 has none of the strict frequency alignment, strict timing control, or signal design features permitting channel estimates from various sources to be determined simultaneously. Therefore, overcoming the limitations of being able to receive multiple 802.11 signals simultaneously using conventional techniques is not practical for 802.11.